The present invention relates generally to a method of and an apparatus for controlling the performance of an exhaust gas purifying device of an engine to a desired value and particularly to a method of and an apparatus for maintaining the temperature of the exhaust gas purifying device within a desired or optimum temperature range in which the exhaust gas purifying device performs most satisfactorily to purify exhaust gases of the engine.
As is well known in the art, internal combustion engines are now provided with an exhaust gas purifying device such as a thermal or catalytic reactor which oxidizes burnable noxious components such as hydrocarbons (HC) and carbon monoxide (CO) contained in exhaust gases emitted from the engine and renders the burnable components into water (H.sub.2 O) and carbon dioxides (CO.sub.2). The production of nitrogen oxides (NO.sub.x) contained in the engine exhaust gases can be reduced by employing a rich or lean air-fuel mixture having an air-fuel ratio lower or higher than a stoichiometric air-fuel ratio. In this instance, the rich air-fuel mixture is usually employed because the lean air-fuel mixture is difficult to ignite and burn in engine combustion chambers. However, by use of the rich air-fuel mixture, the content of the burnable components in the engine exhaust gases is increased to promote the combustion of the engine exhaust gases in the reactor to increase the temperature of the reactor to, for example, above 1,000.degree. C. As a result, there has been a danger that the reactor is heated above its heat resistance limit and is damaged by heat and/or the reactor body is burned and discharged to the atmosphere to cause secondary public nuisance. Furthermore, the use of the rich air-fuel mixture results in increase in the rate of fuel consumption.
On the other hand, it is necessary for combustion of the burnable components in the reactor to maintain the temperature in the reactor above a predetermined level such as, for example, 6,000.degree. C. When the temperature of the reactor is below the predetermined level, the reactor cannot satisfactorily function to purify the engine exhaust gases and allows discharge of engine exhaust gases containing large quantities of burnable noxious components into the atmosphere. Accordingly, when the temperature in the reactor is below the predetermined level as during starting operation of the engine, it is necessary to increase the temperature of the reactor above the predetermined level as quickly as possible.